Also check out Delivery Notification: http://kentweare.blogspot.com/2007/11/biztalk-delivery-notification.html
Becareful with SMTP though. The behaviour that I have experienced is that BizTalk can pass the message off to the mail server(in my case Exchange) but if the mail server cannot not deliver it, BizTalk will not be made aware since it is really just a hand off to the email server. To mitigate some of the risks, have the reply address in your send port configuration an account that is monitored, this way if the mail bounces back at least you will know about it.
Kent